1. Field of the Invention
The present invention relates to a compact and light-weight lens system which can be manufactured at a low cost and has high performance, and more specifically an image pickup lens system which is suited for use in lens shutter cameras and a camera which uses the image pickup lens system.
2. Description of the Prior Art
A zoom lens system which consists, in order from the object side, of a first lens unit having a positive refractive power and a second lens unit having a negative refractive power is conventionally known as a compact image pickup lens system such as a compact zoom lens system which can be manufactured at a low cost.
Lens systems disclosed by Japanese Patents Kokai Publication No. Hei 2-18511, No. Hei 5-224122, No. Hei 6-331899, No. Hei 7-253540 and No. Hei 8-334693 are known as conventional examples of image pickup lens system (zoom lens system) which has the composition described above.
Each of these conventional examples uses a first lens unit which is composed of three lens elements and a second lens unit which is composed of two lens elements, or each of the zoom lens systems consists as a whole of five lens elements. Four or all of the five lens elements are made of plastic materials to manufacture the zoom lens system at a low cost.
A lens system disclosed by Japanese Patent Kokai Publication No. Hei 5-113537 is also known as another conventional example. This conventional example also consists of five lens elements, and uses a plastic lens element having a relatively weak refractive power in a first lens unit and another plastic lens element in a second lens unit so that the lens system can be manufactured at a low cost while restricting focal lengths of the plastic lens elements and so on so as to reduce variations of imaging performance which are to be caused due to changes in environmental conditions such as temperature and humidity.
When plastic lens elements which have sufficiently strong refractive powers are used in a large number in a lens system as in the lens systems disclosed by Japanese Patent Kokai Publication No. Hei 2-18511, No. Hei 5-224122, No. Hei 6-331889, No. Hei 7-253540 or No. Hei 8-334693, the lens system has a defect that it allows its imaging performance to be varied due to changes of environmental conditions such as temperature and humidity. When the imaging performance is varied due to a humidity change in particular, a long time is required until the performance is stabilized, thereby making is difficult to preestimate how the imaging performance, for example an imaging location, will be changed and to correct a change of the imaging location by moving the lens elements. Imaging performance is remarkably degraded when plastic lens elements having sufficiently strong refractive powers are used in a large number as described above.
Furthermore, the lens system disclosed by Japanese Patent Kokai Publication No. Hei 5-113517 which uses only the two plastic lens elements out of the five lens elements cannot be manufactured at a sufficiently low cost.
Furthermore, the lens system disclosed by Japanese Patent Kokai Publication No. Hei 5-224122 uses a cemented plastic lens component in the first lens unit. However, a cemented plastic lens component which consists of two different kinds of plastic lens elements can hardly maintain a stable cemented condition since the plastic lens elements have high coefficients of expansion.